It is known from assignee's own U.S. Pat. No. 4,817,111 to employ a turbine-type compressor in a fast axial flow gas laser for circulating the laser gas in a closed loop through the laser tube at speeds approaching the speed of sound in the laser gas. The impeller of the turbine-type compressor is rotatably supported on a drive shaft of the compressor. Lubricated bearings rotatably support the drive shaft. To avoid contamination of the laser gas with lubricant from the bearings and atmospheric gas, a fluid seal is provided about the compressor drive shaft between the impeller and the bearings to prevent lubricant and atmospheric gas from moving to the impeller along the drive shaft.
The known sealing arrangement disclosed in U.S. Pat. No. 4,817,111 comprises a tungsten carbide mating ring which is sealingly attached to the drive shaft at a location between the bearings and the impeller for rotation with the shaft. A pair of annular, spaced, stationary carbon members with low friction sliding faces are positioned on respective sides of the mating ring and are yieldably biased against the mating ring by springs. The faces of the mating ring adjacent the stationary carbon members each have a spiral groove formed therein. The grooves extend from a location radially outward of the opposed faces of the stationary members to a location between opposed contacting faces of the ring and members.
When a sealing gas is supplied to the seal, rotation of the compressor drive shaft and mating ring thereon causes the spiral grooves in the ring faces to pump the sealing gas between the ring and stationary members which moves the members away from the ring by a small distance. The sealing gas, which may be the same gas used as the laser gas, is supplied to the fluid seal at a pressure slightly higher than atmospheric pressure so that during rotation of the drive shaft the sealing gas flows through the small gap between the opposed faces of the mating ring and stationary members and along the drive shaft to act as a seal against migration of bearing lubricant and atmospheric gas in the direction of the impeller. Sealing gas moving in the direction of the laser is permitted to enter the laser as make-up gas.
In addition to the aforementioned dynamic seal, this known fluid seal also provides a static seal, e.g. when the compressor shaft is not rotating, such as during pumping down of the gas pressure in the laser. The static seal is achieved by means of the faces of the mating ring closely contacting the opposed faces of the stationary carbon members with these components being sealed by O-rings with the shaft and seal housing, respectively. This known fluid seal, which seals during both dynamic and static conditions of the compressor drive shaft, is relatively costly and can present a problem in reliability under certain conditions such as impure gases and fluid and particles. There is a need for an improved sealing arrangement and method for preventing undesirable substances from moving to the impeller of a compressor along a drive shaft of the compressor which overcome these drawbacks of the prior art.